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Top supported balanced draft furnaces are designed with a furnace bottom seal to take care of the downward expansion of the furnace and also to seal the atmosphere from the negative pressure inside the furnace. A very simple arrangement of a water trough with seal plates dipping inside maintains this seal. The level of water in the trough is maintained by a water inlet and overflow system though a pump. Breaking of this water seal causes the induced draft fans in the boiler to become overloaded. This will result in the furnace pressure becoming positive and danger of flame licking out through the openings.
There are many reasons for furnace seal breakage. In a coal fired boiler, a large clinker falling into the bottom hopper can lead to a furnace seal breakage. The large clinker fall results in an appreciable volume of steam production from the water trough of the bottom hopper which pushes out the water from the seal area. The seal water can evaporate due to make up water being low or blocked. When make-up water is inadequate, slowly the level drops and the seal breaks after the level falls below the minimum level. Due to any reason, when a sudden furnace pressure surge happens then the water gets thrown out of the trough breaking the seal.
The plant will respond to the water seal breakage indicating the need for the operator to take corrective action. There will be a large furnace pressure fluctuation depending upon the load, the nature of seal breakage, the duration, etc. The induced draft fan gets overloaded due to the change in furnace pressure and hunting. If the furnace pressure fluctuation is large, then there are good possibilities that the combustion in the furnace also gets disturbed.
The boiler operator will have to restore the furnace seal water immediately. If there is a clinker fall then this has to be analyzed as to why clinkers are falling and resolved. It will be required to look into the wall blower frequency for its effectiveness and adequacy and institute regular wall blowing at the required or revised frequency levels. The combustion regime will have to be studied and tuned to an optimum level if the clinker fall is due the change in fuel quality.
The local operator will have to do a regular check on the water level in the trough and take corrective action to maintain the water level. If it is noticed by the local operator that the level is fluctuating highly, then this has to be informed to the control room and investigated. It is a good practice of the local operator to check the clinker fall from the bottom most peep holes in the boiler, which are normally just above the bottom hopper level.